Inkjet printing mechanisms use pens which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead moves back and forth across the page shooting drops as it moves. To clean and protect the printhead, typically a service station is mounted within the printer chassis. For storage, or during non-printing periods, service stations usually include a capping system which seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead.
During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting." Typically, the waste ink is collected in a stationary reservoir portion of the service station, which is often referred to as a "spittoon." After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide faster, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solid content than the earlier dye based inks, which results in a higher optical density, increased media independence and other advantages for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper. Unfortunately, the combination of small nozzles and quick drying ink leaves the printheads susceptible to clogging, not only from dried ink and minute dust particles or paper fibers, but also from the solids within the new inks themselves.
Partially or completely blocked nozzles can lead to either missing or misdirected drops on the print media, either of which degrades the print quality. Thus, spitting to clear the nozzles becomes even more important when using pigment based inks, because the higher solids content contributes to the clogging problem more than the earlier dye based inks. Unfortunately, while stationary spittoons were suitable for the earlier dye based inks, they suffer a variety of drawbacks when used with newly developed pigment based inks.
For example, FIG. 8, is a vertical sectional view of a conventional prior art spittoon S which has been receiving waste ink of the newer variety for a period of time. The rapidly solidifying waste ink has gradually accumulated into a stalagmite I. The ink stalagmite I may eventually grow to contact the printhead H, which could interfere with printhead movement, print quality, and/or contribute to clogging the nozzles. Indeed, ink deposits along the sides of the spittoon often grow into stalagmites which can meet one another to form a bridge blocking the entrance to the spittoon. To avoid this phenomenon, conventional spittoons must be wide, often over 8 mm in width to handle these new pigment based inks. This extra width increases the overall printer width, resulting in additional cost being added to the printer, both in material and shipping costs.
This stalagmite problem is particularly acute for a polymer or wax based ink, such as an ink based on carnauba wax, or a polyamide. In the past, inkjet printers using polyamide based inks have replaced the conventional spittoon of FIG. 8 with a sheet of flat plastic. The nozzles are periodically cleared by "spitting" the hot wax ink onto the plastic sheet. At regular intervals, an operator must remove this plastic sheet from the printer, flex the sheet over a trash can to remove the waste ink, and then replace the cleaned sheet in the printer. This cleaning step is particularly inconvenient for operators to perform on a regular basis, and is not suitable for the new pigment ink. In comparison to the wax or polymer based inks, these new inks leave a waste which is quite dirty, due to the high amount of solids used to improve the contrast and quality of the printed images, and due to a non-evaporable liquid fraction. Thus, operator intervention to regularly clean a pigmented ink spittoon could lead to costly staining of clothing, carpeting, upholstery and the like.
Besides increasing the solid content, mutually precipitating inks have been developed to enhance color contrasts. For example, one type of color ink causes black ink solids to precipitate out of solution. This precipitation quickly fixes the black solids to the page, which prevents bleeding of the black solids into the color regions of the printed image. Unfortunately, if the mutually precipitating color and black inks are mixed together in a conventional spittoon, they do not flow toward a drain or absorbent material. Instead, once mixed, the black and color inks instantly coagulate into a gel, with some residual liquid being formed.
Thus, the mixed black and color inks have the drawbacks of hot-melt inks, which have an instant solid build-up, and the aqueous inks, which tend to run and "wick" (flow through capillary action) into undesirable locations. To resolve the mixing problem, two conventional stationary spittoons are required, one for the black ink and one for the color inks. As mentioned above, these conventional spittoons must be wide to avoid clogging from stalagmites growing inward from the spittoon sides. Moreover, using two spittoons further increases the overall width of the printer, which undesirably adds to the overall size of the inkjet printer, as well as its weight and material cost to build.
Ink aerosol generation is another problem encountered in inkjet pens. The aerosol problem can be especially severe with pigment based inks at high resolutions, such as those on the order of 600 dpi (dots per inch). Ink aerosol or satellites are micron-sized airborne ink particles, which are generated every time the printhead ejects an ink droplet, both during printing and spitting. Unfortunately, the new inks may need more spitting than dye based inks to refresh the nozzles, due in part to the higher resolutions and the higher solids content of the new inks. Thus, there are more opportunities to generate aerosol when using the new pigmented inks.
The small size and mass of these aerosol particles allows them to float in the air, migrating to settle in a variety of undesirable locations, including surfaces inside the printer. Motion of the printhead carriage generates air currents that may carry the ink aerosol onto critical components, such as the carriage position encoder optics or the encoder strip. Aerosol fogging of the encoder components may cause opacity, as well as light scattering or refraction, resulting in the loss of carriage position information. This migrating ink aerosol may also increasing friction and cause corrosion of moving components, as well as degrading the life of critical components. For example, ink aerosol may accumulate along the printhead carriage guide rod, decreasing bushing life and increasing friction during normal operation.
Worse yet, this aerosol may settle on work surfaces near the printer, where it can then be transferred to an operator's fingers, clothing or other nearby objects. When the pen fires to print an image, many of these extraneous aerosol droplets land on the page, rather than floating around inside the printer. Unfortunately, these extraneous droplets then degrade print quality. Efforts to improve reliability have also contributed to the aerosol problem. For example, low evaporation rate solvents have been employed to address the nozzle clogging problem discussed above. Unfortunately, these solvents cause the aerosol droplets to dry very slowly, if at all, once deposited inside the printer.
New wiping strategies are needed for the pigment based inks to maintain a high print quality in the hardcopy image output. Besides the problems encountered in spitting, new challenges have also arisen in wiping these new inks from the printheads. To maintain the desired ink drop size and trajectory, the area around the printhead nozzles must be kept reasonably clean. Dried ink and paper fibers are known to stick to the nozzle plate, which causes print quality defects if not removed. Wiping the nozzle plate removes excess ink and other residue accumulated along the pen face.
In the past, the printhead wipers have been a single or dual wiper blade made of an elastomeric material. Typically, the printhead is translated across the wiper in a direction parallel to the scan axis of the printhead. In one printer, the wipers were rotated about an axis perpendicular to the printhead scan axis to wipe. Today, most inkjet pens have nozzles aligned in two linear arrays which run perpendicular to the scanning axis. Using these earlier wiping methods, first one row of nozzles was wiped and then the other row was wiped. While these earlier wiping methods proved satisfactory for the traditional dye based inks and for slower drying pigment inks, unfortunately, they are unacceptable for the newer fast drying pigment inks.
In using the fast drying pigment based inks, three primary failure modes were discovered using traditional wipers. First, the ink dries out, and then sticks tightly to the nozzle plate with such a force that a traditional wiper cannot move the ink, even through the use of high force wipers. Unfortunately, high force wipers risk damaging the printhead, and they require a heavier base structure to support the wiper. In the second failure mode, dried ink particles occasionally broke loose and were then rolled up by the wiper. Unfortunately, these ink rolls often settled over a nozzle, causing a partial or total blockage interrupting ink ejection. In the third failure mode, the ink would dry out in layers around a nozzle in a shape resembling a volcano caldera, which then caused drop trajectory problems. Traditional wipers were not able to effectively remove the dry ink down to the caldera base, which resulted in formation of caldera over time.